pinctrl: microchip-sgpio: wait until output is actually set

Right now, when a gpio value is set, the actual hardware pin gets set
asynchronously. When linux write the output register, it takes some time
until it is actually propagated to the output shift registers. If that
output port is connected to an I2C mux for example, the linux driver
assumes the I2C bus is already switched although it is not.

Fortunately, there is a single shot mode with a feedback: you can
trigger the single shot and the hardware will clear that bit once it has
finished the clocking and strobed the load signal of the shift
registers. This can take a considerable amount of time though.
Measuremens have shown that it takes up to a whole burst cycle gap which
is about 50ms on the largest setting. Therefore, we have to mark the
output bank as sleepable. To avoid unnecessary waiting, just trigger the
single shot if the value was actually changed.

Signed-off-by: Michael Walle <michael@walle.cc>
Link: https://lore.kernel.org/r/20220226204507.2511633-6-michael@walle.cc
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>